In receiving and transmitting unit of wireless communication systems, filter circuits are generally used so as to filter signals within a range of bands other than a predetermined signal band. There are various kinds of filter circuits. However, among them, filter circuits whose time constant (cutoff frequency) is determined by multiplication between a resistance value (R) and capacitance value (C) are effective in integration. Therefore, these filter circuits have been used for integration circuits in wireless communication terminals, such as cellular phones or the like.
However, a problem may occur in the filter circuit constructed on the integrated circuit due to the following reason. Since variation occurs in manufacturing elements, a time constant may vary, which results in lowering a manufacture yield. As an example of a method of preventing a manufacture yield from being lowered due to the manufacture variation, as in a method described in “An Ultra Low-Voltage Gm-C Filter for Video Applications”, S. Mehrmanesh, et. al., ISCAS2003, I-561-564, a clock is input to a test filter circuit, and a time constant is automatically adjusted such that an amount of shifted phase of a clock in the output of the test filter circuit becomes a predetermined value.
In this method, according to the output results of two comparators, an up-down counter allows its output bit to be upward or downward, repeats the up or down operation until output results of the phase comparison circuit become predetermined values, and adjusts phase variation in the filter circuit, that is, a time constant.
However, in this circuit structure, when the element variation is large, the number of times of the repeated up/down operation increases, which results in an increase of time taken until the adjustment of the phase variation is completed. As a result, an amount of consumed current increases as much as the increase of the time taken until the adjustment of the phase variation is completed. In addition, when the circuit structure is used in a system in which the short adjustment time is required, the manufacture yield may be lowered, which causes a problem.
According to the above-described related art, much adjustment time is required when automatically adjusting a time constant of a filter circuit in an integrated circuit, which results in increasing a power consumed by the integrated circuit or lowering a manufacture yield of the integrated circuit.